You're lucky it is just recorded shows for you. I have the same problem with "live" shows. If I "rewind" the sound is there, so I know it is getting down the line OK. It just can't seem to get through the box. So far Verizon has shown NO interest in this problem. I can't even get the techs to acknowledge it exist.
Press the Menu button on your remote and find the Customer Support option and Fix Common DVR Issues. If you run this app, it will scan your DVR and correct filesystem errors. It will reboot your DVR (perhaps a couple of times). The process takes about 5 - 10 minutes. Verizon keeps moving this item around and calling slightly different names. It is not uncommon to have to run this utility once every few weeks since the DVRs seem to get corrupted or freaked out by any little system change or channel issue that happens.
This also works for black screens with or without audio, channel switching issues. stuttering issues and sometimes will temporarily fix pixelation issues. Pixelation issues usually are a sign the DVR hard drive is failing though.